1. Field of the Invention
The present invention generally relates to electrophotographic printers. More particularly, it relates to apparatus and techniques for improving the print quality of such printers by preconditioning the print media with heat and/or pressure.
2. Prior Art
Electrophotographic printing processes generally comprise the steps of using a laser beam to form an electrical latent image on a charged photoconductor drum, developing that latent image with a toner, transferring the resultant toner image onto a transfer substrate such as a sheet of paper and then fusing the toner image to the transfer substrate by means of heat, pressure and the like. Those skilled in this art will appreciate that the toner can be particulate or liquid in nature. In either case the toner fusing operation becomes especially involved in the case of electrophotographic color printing processes wherein images employing various colors, e.g., cyan, yellow, magenta and black (C, Y, M, K) toner particles are successively printed on a photoconductive drum and then transferred from the drum to a print media. In such processes, each successive color image printed on the sheet of print media (e.g., paper) is usually individually fused thereon.
It is well known that the quality of electrophotographic printing processes can vary with changes in ambient temperature and relative humidity. Print quality also can vary as the electrophotographic printer itself heats up during periods of extended use. These variations are generally related to the moisture content of the individual sheets of print media undergoing electrophotographic printing. For example, it is well known that the physical size of a sheet of paper (and especially bond paper) can vary with the paper""s moisture content. A standard sheet of 8xc2xdxc3x9711 inch bond paper, for example, can shrink as much as a quarter of an inch in either dimension as it goes through an electrophotographic printer""s pressure/fuser assembly. Such changes in paper size are sometimes annoying to a reader. They can be especially annoying in duplex printing situations where a sheet of paper receives printed information on a first side and then undergoes a heating and pressing operation that fuses the toner to the paper. This heating and pressing operation shrinks the paper. The sheet of paper then undergoes a duplex printing operation wherein the second side of the sheet receives printed information. In this circumstance the printed information on the second side of some kinds of paper will tend to xe2x80x9cshow throughxe2x80x9d the paper at the borders of the first side. This condition can create visual effects that vary from reader annoyance, to unprofessional appearance, to commercial unacceptability.
Those skilled in this art also will appreciate that, in order to carry out a toner transfer, the print media passes between a toner transfer roller and the photoconductor drum. During the toner transfer, the transfer roller electrostatically attracts toner away from the surface of the photoconductor drum and onto the surface of the print media (e.g., a sheet of paper). The electrical resistivity of the print media is one of the many factors involved in this electrostatic transfer of the toner from the drum to the media. The sheet""s electrical resistivity is, in turn, especially effected by the moisture content of the media. Therefore, one of the primary objects of this invention is to precondition each sheet of print media (e.g., each sheet of paper) in a substantially uniform manner so that successive sheets of that print media will have virtually the same moisture content (and hence virtually the same electrical resistivity and hence the same electrostatic properties) as they pass between the transfer roller and the photoconductor drum. This uniformity of electrostatic properties of each successive sheet of print media helps to provide more uniform and, hence better, electrophotographic printing results.
This invention is particularly concerned with the use of pressure/heater devices to pretreat successive sheets of media (e.g., successive sheets of paper) that undergo electrophotographic printing processes. This pressure/heat pretreatment can be introduced into an otherwise conventional electrophotographic printing process. It takes place prior to the point in such a process where the toner image is transferred to the paper. The herein described pretreatment process serves two purposes. It preshrinks successive sheets of print media (e.g., paper) to a uniform size. Thus, it serves to minimize subsequent sheet shrinkage after the toner has been applied to one side of the paper and then duplexed. Applicant""s pressure/heat pretreatment also serves to provide successive sheets passing through an electrophotographic printing apparatus with substantially the same moisture contentxe2x80x94and hence substantially the same electrostatic properties. This is especially useful in uniformly transferring successive toner images from the photoconductor drum to successive sheets of paper as said sheets of paper pass between the photoconductor drum and the transfer roller.
The electrophotographic printing apparatus of this patent disclosure is especially characterized by the fact that it is provided with a pressure/heater device that is preferably comprised of two opposing rollers that roll over each other in pressured, rolling contact. In some of the preferred embodiments of this invention, at least one of the two opposing rollers will contain a heating device such as an inductive heater element or a halogen tube. Use of two opposing rollers wherein each of the two opposing rollers contains a heating device is also contemplated in the practice of this invention. Use of a powered heater roller also is contemplated. Use of two separately powered rollers is also possible, but not preferred.
The temperature and pressure conditions existing in the pressure/heater devices of this patent disclosure (e.g., pressure heater device 54/56 of FIG. 2 and/or pressure heater device 46/48 of FIG. 3) can vary considerably. They can vary with respect to each other and they can vary with respect to the residence time of a sheet of print media (e.g., a sheet of 8xc2xdxc3x9711 inch paper) in said pressure/heater devices. Generally speaking, the temperature of the roller surface of the heater roller should serve to soften (but not melt) a toner material. Generally speaking, such temperatures may vary between about 150 and about 350xc2x0 F. Temperatures between about 250 and 350xc2x0 F. are however somewhat preferred in those cases where polymer based toner particles are being employed in the inkjet printing process. The pressure conditions experienced by a sheet of media, and especially a sheet of paper, will generally range between about 232 and about 472 psi. Pressures between about 400 and about 472 psi are preferred, especially when the heater roller temperature is between about 329xc2x0 F. and about 374xc2x0 F.
The residence time of a sheet of media in a pressure/heater device of this patent disclosure is largely determined by the angular velocity of a powered drive roller. Typical residence times for an 8xc2xdxc3x9711 inch sheet of paper will be from about 2 to about 8 seconds. Residence times of about 3 to about 6 seconds are more preferred. These preferred residence times generally correspond to 8xc2xdxc3x9711 inch paper processing rates of about 16 to about 32 sheets per minute. Generally speaking, the shorter residence times will be used as the operating temperature is raised. For example, the lower end of the residence time range (e.g., 2-3 seconds) will generally be preferred as the temperature is raised to the upper regions of its range (e.g., 250-350xc2x0 F.).
Thus, the electrophotographic printing apparatus of this patent disclosure will preferably comprise: (1) a laser printing device for creating a latent image on a photoconductor drum, (2) a sheet transport system leading to a toner transfer zone, (3) a photoconductor drum whose outside circumferential surface defines a top side of the toner transfer zone, (4) a transfer roller whose outside circumferential surface defines a bottom side of the toner transfer zone, and wherein the sheet transfer system leading to the toner transfer zone further comprises a (5) sheet pretreatment device for providing pressure and heat to successive sheets in order to provide said sheets with substantially the same moisture content and, hence, substantially the same electrostatic properties. Such an electrophotographic printing apparatus will preferably have two opposing rollers that create a nip and wherein at least one of the two opposing rollers contains a heating device such as an inductive heater element or halogen tube. In other, less preferred, embodiments of this invention, the electrophotographic printing apparatus pretreatment device may have two opposing rollers that each contain a heating device and its own means of powered rotation.
The apparatus and methods of this patent disclosure are especially well suited to electrophotographic printing processes wherein the sheet pretreatment process and the toner fixing step are carried out by the same pressure/heater (fuser) device. For example, such an electrophotographic apparatus might comprise: (1) a laser device for creating an image on a photoconductor drum, (2) a toner hopper for storing and dispensing toner particles on to the photoconductor drum, (3) a first sheet transport system that leads from a sheet dispenser tray to the pressure/heater device and then to an internal sheet collection tray (which may also help perform a duplexing function), (4) a second sheet transport system that carries a sheet from the internal sheet collection tray to the toner transfer zone, (5) a photoconductor drum whose outside surface defines a first or top end of the toner transfer zone, (6) a transfer roller whose outside surface defines a second or bottom end of the toner transfer zone, (7) a second sheet transport system that leads from the toner transfer zone to the pressure/heater (fuser) device and (8) a pressure/heater (fuser) device having a first mode of operation for providing successive sheets with substantially the same moisture content and a second mode of operation for fixing the toner to the sheet.